The present invention relates to transfer mechanisms, in particular to a feed-screw type transfer mechanism, for transferring an optical pick-up device used in an optical disc drive.
Conventionally, so-called “feed-screw type” transfer mechanisms have been commonly used for transferring an optical pick-up device used in an optical disc drive (see Patent Literature 1: Japanese Patent Laid-open Publication No. 2005-93037, for example). As have been well known, a typical feed-screw type transfer mechanism includes a drive shaft provided with an external screw around its external circumference and a follower member having teeth that are engaged with the external screw, and the mechanism is configured such that driving the drive shaft to rotate by a predetermined amount using a motor feeds the follower member by a predetermined amount along a longitudinal axis of the drive shaft, and whereby an optical pick-up device is transferred in the same direction by a predetermined amount in association with the follower member.
However, according to the conventional configuration disclosed in Patent Literature 1 (Japanese Patent Laid-open Publication No. 2005-93037), as illustrated in FIG. 9, the optical pick-up device transfer mechanism is provided with a feed screw 10 (drive shaft) configured as a common single-thread screw and having a screw portion 10a around its external circumference, and a teeth portion (follower member) 23 including three teeth 23a that are engaged with the screw portion 10a. In order to transfer an optical pick-up device coupled to the follower member 23 at a high speed and with high accuracy by engaging the three teeth 23a of the follower member 23 with the screw portion 10a of the drive shaft 10, processing and finishing with very high accuracy are required for such as dimensions, sizes, and surface finish of the screw portion 10a of the drive shaft 10 and the three teeth 23a of the follower member 23. This disadvantageously results in an increased cost.
Further, conventionally, as illustrated in FIG. 8, for example, an external screw having a relatively large feed pitch (3-mm pitch, for example) has been generally used as a screw portion 242 of a drive shaft 240 in order to realize high speed transfer. Therefore, a circumferential surface portion 242c of an outer circumference provided between adjacent thread grooves 242g and 242g (a flat portion of an outer circumference in a cross-section of the drive shaft) becomes relatively large. Accordingly, when an impact load is applied to an assembly of the drive shaft 240 and a follower member 250 due to, for example, a fall during distribution of the disc drive or handling of a component unit including an optical pick-up device transfer mechanism, there is a problem that a so-called “running over of teeth” phenomenon, in which teeth 256 of the follower member 250 fall out of the thread grooves 242g of the screw portion 242 of the drive shaft 240 and run over the circumferential surface portion 242c of the outer circumference, can easily occur. Once this phenomenon occurs, it is difficult to recover to a previous state, and the optical pick-up device cannot be transferred.
In order to address such a “running over of teeth” phenomenon, as illustrated in FIG. 7, for example, using a double-thread screw as a feed screw 142 provided over an external circumference of a drive shaft 140 can be conceived. As have been well known, the double-thread feed screw 142 is configured by threading an additional single-thread screw 144 separately provided between the threads of a single-thread screw 143 at the same feed pitch. Using the double-thread screw 142 thus configured can decrease a circumferential surface portion 142c of an outer circumference provided between adjacent thread grooves 143g and 144g as compared to a common single-thread screw (an axial length can be reduced substantially to half), and is highly effective in reducing the occurrence of the “running over of teeth” phenomenon.
In the meantime, in the feed-screw type transfer mechanism, in order to move the follower member smoothly and at a high speed along the longitudinal axis of the drive shaft, suitable lubrication of a contact portion between the external screw around the external circumference of the drive shaft and the teeth of the follower member is essential. Therefore, a lubricant such as grease is applied to the screw portion of the drive shaft. However, as such a lubricant is usually applied to the screw portion of the drive shaft using a brush or a dispenser, it is extremely difficult in practice to apply the lubricant only to a desired portion so that the lubricant uniformly spreads over the desired portion, especially when a multiple-thread screw, for example, a double-thread screw, is used for the external screw of the drive shaft.
Describing with reference to the FIG. 7, with regard to the screw 143 that is to be engaged with teeth 156 of a follower member 150 (i.e., a screw relating to transferring), in a shipping inspection and the like, rotation of the drive shaft 140 by a certain number causes the follower member 150 to move along the longitudinal axis of the drive shaft 140, and this causes the teeth 156 of the follower member 150 to relatively move along a thread groove 143g of the screw 143 of the drive shaft 140. With this, a lubricant G applied to a feed screw portion 142 of the drive shaft 140 that has been initially non-uniformly pooled in the thread groove 143g spreads thin along the thread groove 143g of the drive shaft 140 uniformly within a range of the movement of the follower member 150.
By contrast, with regard to the screw 144 that is not to be engaged with the teeth 156 of the follower member 150 (i.e., a screw not related to transferring), the lubricant G (e.g., grease) is non-uniformly pooled in the thread groove 144g in an initial state of the application. There is a problem that the lubricant G pooled in the thread groove 144g may spread out of the thread groove 144g and scatter around along with high speed rotation of the drive shaft 140, and then stick to a lens of an optical pick-up device and an optical disc (all of these are not depicted), resulting in a tremendous adverse effect on quality in recording and/or reproduction by the optical disc drive.
As a countermeasure to this problem of scattering of the lubricant, providing a protective cover capable of shielding the optical pick-up device transfer mechanism from the optical disc and the optical pick-up device is conceivable. However, in the case in which such a countermeasure is employed, there are problems of making a structure around the optical pick-up device transfer mechanism highly complicated and considerably increasing the cost.